Whether it’s fascination, fear or a mixture of both, sharks capture our imagination. Hollywood hysteria aside, there are a lot of things they do to keep our attention. Sharks can be bulls or tigers. They can be hammers or cookie cutters. Some of the stranger ones can even be bioluminescent ghosts, while others that even can reproduce asexually.
But were sharks always this diverse? What were the earliest sharks like, and when did some sharks come to dominate the marine food chain?
It depends on how far you want to go back of course, but some researchers now believe that the group that includes all three living groups of sharks — rays, sharks and chimeras — evolved first from the acanthodians.
The beginning of sharks: the acanthodians. (Credit: Serafima Antipova/Shutterstock)
“They are all terribly spiny and small,” says Michael Coates, a biologist at the University of Chicago. “Nonetheless, they show us the root of sharkdom — getting a bony interior skeleton.”
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Characteristics of Acanthodians
When acanthodians began to develop, they weren’t the intimidating sharks we see today. They didn’t have retractable teeth like the great white or the enormous size of the megalodon — these traits would take millions of years to develop.
Most acanthodians were fingerling-sized — just anchovies in size. They were likely among the first fishes that moved off the seafloor into the water area during the so-called nekton revolution in the Silurian and Devonian periods.
As with other organisms adapting to life in open water for longer periods, this shift upwards involved a lot of new changes — things like larger eyes and multiple paired fins evolved to stabilize their swimming. The earliest sharks also had fewer bony plates in their skulls and reduced scales.
From Acanthodians to Chimeras
By the Devonian period, when the Chondrichthyes that include the crown groups of the elasmobranchs (rays and sharks) and chimeras evolve from the acanthodians, they no longer had internal bones but cartilage, which is metabolically easier to produce and neutrally buoyant.
“You’ve got to up your motor system, you’ve got to up your sensors,” Coates says.
Other characteristics common to the males in this crown group include the pelvic clasper — “effectively a paired penis,” Coates says. Specialized teething systems began to appear, including rapid tooth shedding and replacement.
“It’s a brilliant simple modular system, it’s very elegant,” Coates says of the teeth system.
Shark females, for their part, began to develop some of the specialized ways of reproduction we see today in the Paleozoic, which includes the internal fertilization that occurs in all sharks, rays and chimeras.
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An Explosion of Ghost Shark Offshoots
The anchovy-like-era doesn’t last forever for sharks. Sometime before the series of mass extinctions that occurred in the Late Devonian, starting roughly 372 million years ago, sharks began to evolve in multiple new and interesting body shapes. Much of the explosive evolutionary radiation in this period occurred among the group that includes chimeras or ghost sharks.
The Symmoriids, probably early offshoots of the evolving chimeroid group, include a whole group of fishes. They were “weird to look at” sharks with strange appendages sticking off of their top and long tail fins.
One group, the Iniopterygians have wing-like fins unlike any other sharks alive today, Coates says. By the late Devonian, some of these sharks began to grow in size — some to lengths of more than two meters long.
But they still aren’t the kings of the sea yet. Dunkleosteus, a genus from a class of armored fish called the placoderms, reached greater dimensions and could have easily preyed on early chondrichthyans.
“You get an idea of who is the apex predator there, and it’s not the sharks,” Coates says.
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The Prolific Evolution of Sharks
Before the end of the Devonian, sharks continued to grow in both abundance and diversity in a way they never would again. The successive extinctions reduced numbers and diversity greatly, but in the subsequent Carboniferous period, shark diversity continues to be dominated by early chimeras and side branches of the chimeras.
While modern ghost sharks are mostly relegated to the depths of the ocean, the order was all over the place back then, with some species even spending time in brackish water, rather than seawater.
“Chimeras are just wonderfully bizarre,” Coates says.
The earliest elasmobranchs — sharks and rays — also began to adopt sophisticated feeding techniques. Some dating to roughly 340 million years ago had developed the ability to suction feed rather than just engulf food with their jaw, similar to modern nurse sharks. “These are the first really effective vertebrate suction feeders that we have on the planet,” Coates says.
It isn’t until much later in the early Miocene that the largest of all sharks, the Megalodon, appears, and shark domination of the sea is complete.
In the span of a few hundred million years, the group of fish had gone from anchovy-sized body feeders to giants measuring up to 20 meters long, by some estimates.
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Source : Discovermagazine